Pistons adapted for use in axial piston pumps and motors as in the aerospace industry are often subject to significant stress concentration levels. That stress typically arises because the pistons reciprocate at high velocities and simultaneously rotate in relation to piston bores. In certain applications, such as an aircraft integrated drive generator (IDG), the relationship of each piston to its respective piston bore within the cylinder block should be controlled within a tight diametral clearance range, such as 0.0001" to 0.0004". In such arrangements, oil is pumped into and out of the piston bore at up to approximately 6000 psi. Each cylinder block of the IDG typically contains nine pistons, and each IDG typically contains four cylinder blocks per aircraft engine. Thus, to increase service reliability of the aircraft and reduce periodic maintenance, it is desirable to minimize wear of the pistons on associated cylinder block bores.
In order to withstand harsh, aircraft operating environments without requiring excess weight, previous pistons have been constructed from a strong, wear resistant material, such as steel, with a hollowed section. One example is disclosed by Havens in U.S. Pat. No. 3,319,575. However, the Havens patent design does not adequately account for certain problems of wear on the piston bore surface caused by "cocking" of the piston. This arises where external forces applied to the piston cause the piston to move laterally against the piston bore, despite the presence of lubricating oil within the piston bore and the use of a copper-based alloy bushing or liner for the piston bore surface.
Cocking can present significant problems because stress concentrations increase where the piston is rubbing against the piston bore surface. This causes the piston to gradually wear away at the piston bore surface and eliminate the performance-based tight tolerance. Often, this can leave a wear path in a range of 0.012"-0.020" deep with the worn material contaminating the lubricating oil. Such extra clearance in the piston bore triggers excess fluid leakage in the cylinder block and piston assembly which can result in low charge pressure and prevent electrical performance frequency ratings of the IDG from being reached. Eventually the wear can cause the hydraulic unit to completely malfunction.
An additional problem resulting from low charge pressure is cavitation. Cavitation results from the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure. Cavitation also appears to have a detrimental effect on piston bore wear; although, the correlation between cavitation and piston bore wear is not fully understood. However, the present invention attempts to separate this relationship between piston bore wear and piston bore cavitation damage.
Accordingly, objects of the present invention include providing an improved method for reducing wear on a piston bore surface caused by external forces acting on a piston, and an advanced piston design and a cylinder block assembly incorporating the advanced piston design which has advantageous wear reduction characteristics. Other objects of the invention include the following:
(i) to maximize efficiency of the piston design with an improved geometry for allowing oil to lubricate the piston bore surface; PA1 (ii) to maximize efficiency of the cylinder block assembly design; PA1 (iii) to reduce contact stress concentration between the piston and the piston bore surface; PA1 (iv) to prevent cavitation within the piston bore of the cylinder block as the piston operates; and PA1 (v) to prevent contamination of lubricating oil caused by worn bushing and piston bore surface material.